Why are plants green rather than red (or black)?

21042009

Plants generate the life in the biosphere by absorbing incoming photons from the Sun – light. Plants are mostly green, and it means they do not absorb the green color. Rather than this, the pigments that govern the photosynthesis absorb mostly on red and blue colors. The ubiquitous green color of plants is testimony to the key molecular participant in the light harvesting of plants, chlorophylls. Also widespread, but less visible, is a second participating molecule, carotenoid. In green leaves the color of the carotenoids is masked by the much more abundant chlorophylls while in red ripe tomatoes or petals of yellow flowers, the carotenoids predominate. Look at the solar spectrum here, in terms of energy the peak flux is around 480-490 nm, which is in the blue-green. But see the gap in the absorption in the green and yellow in the clorophyl and carotenoid pigments in Fig. 2 here.

This seems odd, and this question has now been generally analyzed by a team of scientists including members of NASA Goddard Institute. According to Drake equation there are some 6.25 billion life-supporting solar systems in the universe. A main marker of photosynthetic life that can be detected astronomically is the distinct reflectance spectra of photosynthetizer organisms. But the pigments will be evolved to optimice absorption of the particular radiation impinging on their enviroment, and Kiang et al. propose some rules for where photosynthetic pigments will peak in absorbance: a) the wavelength of peak incident photon flux; b) the longest available wavelength for core antenna or reaction center pigments; and c) the shortest wavelengths within an atmospheric window for accessory pigments. They conclude: “That plants absorb less green light may not be an inefficient legacy of evolutionary history, but may actually satisfy the above criteria.”

A systematic gathering of data of photosynthetizer organisms, presented in paper I cited below, shows some very general trends. The first, important for organic solar cell research, is that plant pigments absorption peaks not at the maximum energy point of the solar spectrum (this is at the green), but at the maximum point of photon flux at the Earth’s surface, which occurs at 685 nm (just before a drop in atmospheric transmittance due to oxygen at 687.5 nm). Thus, the peak absorbance at the left side of the red (red color is classified as between 620–750 nm) is an adaptation to harvesting light in the atmospheric transmittance window with the most abundant photon flux. In addition, the peak is at the most red-shifted limit of this window, to afford exciton transfer (by resonant energy transfer) from accesory pigments.

The origin of the difference between the photon flux and energy flux, is that photons at shorter wavelength carry more energy. So less photon density in the blue-green is able to contain more photon energy. However, in energy conversion in photosynthesis, as well as in solar cells, the photons are converted one by one. Provided that each photon has enough energy to conduct the required reactions, all photons count the same. It makes total sense that plants evolved to harvest the photons in the frequency range where they are more abundant.

The reflectance spectra of five materials is shown in the next Figure*, Note that reflectance of vegetation decreases in the red (where they absorb), and peaks in the green (the “color” of plants).

Another strinking phenomenon that occurs quite generally is a sharp rise of the reflectance in the near infrared (NIR). It means that the plants reject all photons with wavelengths longer than 700 nm (see a picture here, which is the spectral reflectance characteristics of different crops at distinct growth phases). These lower energy photons, should be not useful to conduct the desired energy storing reactions, but it seems misterious why the plants avoid absorbing these photons. Although the NIR must play some role in the organism’s energy balance, it is far from clear how important this is in the organism’s survival.

And indeed with the peak absorbance in the red, green light is wasted!

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Heraeus and BELECTRIC presented at expo Milan robust and efficient organic solar cells that can be integrated into tree-like structures creating a large canopy. The effect is an attractive, futuristic architecture that generates electricity. This style of OPV can be used to generate solar power from almost any surface. Dr. Antonio Guerrero holds the organic […]